Conventional ion-exchange resins and composite adsorbents have suffered from a number of disadvantages.
First, there has been the risk of interaction between the magnetic particles and the components of the system in which the ion exchange resins or composite adsorbents have been employed. This is a particularly serious problem with the porous composite adsorbents referred to above. Many of the common magnetic materials, such as ferrites, cannot be readily used at a pH below 7 and are rapidly degraded at a low pH.
Secondly, it has been very difficult to avoid "flushing" of the particulate matter during the preparation of the resin or composite adsorbent bead. "Flushing" refers to the tendency of the particle to leave the polymer during the formation of the bead. The particles frequently are competely excluded from the forming bead or concentrate and protrude from the outer surfaces of the bead. Where attempts have been made to encapsulate magnetic particles in a protective coating prior to incorporation in a resin or composite adsorbent, flushing has also been a problem.
A further problem relates to the processes of manufacturing ion exchange resins. Frequently it is desired to prepare polymeric beads and by subsequent reaction schemes to introduce onto these beads functional groups which will provide the desired ion exchange function. When magnetic particles are incorporated into the beads the range of possible subsequent chemical reactions of the polymeric bead matrix can be severely limited by the susceptibility of the magnetic material to attack. For example, for the important class of cross-linked styrene based resins key processes such as chloromethylation and sulphonation with chlorosulphonic acid or oleum cannot be used.